Identification system



Jan- 9, 1968 B. P. cI-IAUssE ETAL 3,363,247

IDENTIFICATION SYSTEM .Filed May 5, 1967 6 Sheets-Sheet l STORAGE REGISTER DIGITAL LOGI C Y TRANSMITTER RECEIVER SWITCH SEOUENCING ISCRIMINATOR MULTIPLI-:x

OSCILLATOR AMPLIFIER INVENTORS BURNETTE P. CHAUSSE RONALD E. GAREIS BY 761% 0f ATTORNEY Jan. 9, 1968 B. P. cHAUssE vETAL 3,363,247

IDENTIFICATION SYSTEM 6 Sheets-Sheet 2 Filed May 5, 1967 S4 amm mmm .G wp. EE mm uw BR @n w E n@ mq @Q @9 m9 v @w O e N |T ATTORNEY Jan. 9, 1968 B. P. cHAussE ETAL 3,363,247

IDENTIFICATION SYSTEM Filed May 5, 1967 6 Sheets-Sheet 5 IS),l

STORAGE REGlSTER Jan. 9, 1968 Filed May 5, 1967 B. P. cHAUssE ETAL 3,363,247

IDENTIFICATION SYSTEM 6 Sheets-511e??l 4 j rg 4A RETRACE SWEEP DOWN IN FREQUENCY :roms FIG 4F SBOKC l l n #agrlu vh BURNETTE P. CHAUSSE RONALD E. GAREIS ATTORNEY INVENTORS Jan. 9, 1968 B. P. cHAUssE ETAL 3,363,247

IDENTIFICATION SYSTEM Filed May 5, 1967 6 Sheets-Sheet. 5

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BY /mw INVENTORS ATTORNEY JaiL 9, 1968 B. P. CHAUSSE ETAL 3,363,247

IDENTIFICATION SYSTEM Filed May 5, 1967 6 Sheets-Sheet 6 INPUT T z-g 5 A 5: 'I 55' OUTPUT :1L-:g5 c.

2T- 5 D. SIGNAL PLUS '-5 NOISE OF 5c OUTPUT EF-E 5E. FROM 5D BURNETTE P. CHAUSSE RONALD E. GAREIS ATTORNY United States Patent O 3,363,247 IDENTEICATION SYSTEM Bumette P. Chausse and Ronald E. Gareis, Roanoke, Va., assiguors to General Electric Company, a corporation of New York Filed May 5, 1967, Ser. No. 641,411 8 Claims. (Cl. 343-65) ABSTRACT F THE DISCLOSURE An object identification system has a unique identification device associated With each object to be identified. A plurality of piezoelectric elements are in each identification device, each element having a different preselected frequency response. Signals are transmitted over a preselected frequency range from a transmitting antenna and signals at the same frequencies, as the piezoelectric elements connected in the identification device, are refiected by the identified device and transmitted to a receiving antenna. The transmitting sweep frequency is only transmitted during the time that there can be frequency response from a piezoelectric element in the identification device, the transmitted sweep signal is then turned off after it passes through each specific frequency at which there is a piezoelectric element having preselected frequen-cy response. Each piezoelectric element continues to oscillate after the exciting transmitted sweep signal is turned off and decays. Signals at the same frequencies of the piezoelectric elements connected in the identification device are reflected during the decay period, and transmitted to a receiving antenna. The receiver is turned on after the sweep si-gnal is transmitted to receive the reflected signals during the decay period from the selected piezoelectric elements in the identification devices. A second sweep signal is generated which is a predetermined frequency different from the transmitted frequency. The second sweep signal and the refiected signals from the identification device are both applied to a discriminating circuit so that the discriminating circuit produces a predetermined difference. A proper identication signal from the identification device causes the discriminating circuit to produce a predetermined frequency signal which is the difference between the second sweep signal and the refiected signal from the identification device. The difference frequency signal is applied to a detection circuit which determines if the difference signal is within predetermined limits to decide if the difference signal is a proper frequency.

This invention is particularly useful in that type of identification system for identifying objects, where each object has a unique identification device associated therewith. In such identication systems the unique identification device associated with each object uses piezoelectric elements, each piezoelectric element having a different preselected frequency response, whereby identification of the object is made by the piezoelectric elements connected in the unique identification device. Signals are transmitted over a preselected frequency range from a transmitting antenna. Signals at the same frequencies as the piezoelectric elements connected in the identification device are reflected by the identification device, transmitted to a receiving antenna, and decoded to indicate the identity of the object as indicated by the identification device.

Identification systems constructed according to this construction have been very successful. `Different aspects of such an identification system have been described in several patents. Patents 3,169,242, 3,209,350, 3,209,351 and 3,210,759, and application Ser. No. 596,376 filed Oct. 25, 1966, all describe such an identification system.

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The only limitation of such identification systems has been that the identification devices must be spaced relatively close to the transmitting and receiving antennas, in order to properly identify the identification device on the object to be identified. The spacing should be approximately one foot in order to properly identify the identification device, and while this may be satisfactory in some applications, in other applications such as railroads, automobiles, and the like, such close spacing may cause problems. yOn railroad cars Wheels Wear, so that the separating distance may vary with the wheel wear. The trucks of railroad cars may also vary in height, depending on whether they are loaded or unloaded, a distance of six to eight inches. A separating distance of twelve inches to start out with could therefore vary between six and eighteen inches. At the higher distance of eighteen inches, noise may be such that it would cause false identification. Thus the separating distance becomes critical, so that applications of such systems become rather limited. The identification devices in such identification systems are also subject to damage from objects projecting from the trackway, or the highway.

In such identification systems, the assigned frequencies of the piezoelectric elements in the identification device are normally l0 kc. apart. Each different frequency response represents a binary bit with the presence of a piezoelectric element representing a binary one, and the absence representing a binary zero. The number of different binary bits necessary in an object identification system depends on the number of objects to be identified, with more binary bits needed as more objects are identified. The tolerances in the piezoelectric elements used in such systems are such that a piezoelectric element rated to respond at 470 kc. may actually respond during a frequency range of 466 kc. to 474 kc.

Noise is also a constant factor that must be considered and is a frequent source of errors in the identification system using piezoelectric elements. It is impossible to eliminate all sources of noise.

'It is therefore an object of this invention to provide a new and improved object identification system.

It is another object of this invention to provide a new and improved object identification system which is more accurate than previous identification systems.

According to this invention therefore, the transmitting sweep frequency is only transmitted during the time that there can be a frequency response from a piezoelectric element in the identification device. The transmitted sweep signal is then turned off after it passes through each specific frequency at which there is a piezoelectric element having a preselected frequency response. Each' piezoelectric element continues to oscillate after the exciting transmitted sweep signal is turned off and decays during a period of microseconds. Signals at the same frequencies of the piezoelectric element connected in the identification device are reflected during the decay period, and transmitted to a receiving antenna. The receiver is turned on after the sweep signal is transmitted to receive the refiected signals during the decay period from the selected piezoelectric elements in the identification device.

A second sweep signal is generated which is a predetermined frequency different from the transmitted frequency. The second sweep signal and the refiected signals from the identification device are both applied to a discriminating circuit so that the discriminating circuit produces a predetermined difference. A proper identification signal from the identification device causes the discriminating circuit to produce a predetermined frequency signal which is the difference between the second sweep signal and the reflected signal from the identification device. The difference 3 frequency signal is applied to a detection circuit which determines if the difference signal is within predetermined limits to decide if the difference signal is a proper frequency.

Description of the drawings In the drawings:

FIG. 1 is a block diagram of an object identification constructed according to this invention.

FIG. 2 is a circuit showing the discriminator circuit.

FIG. 3 is a block diagram of the detector circuit.

FIGS. 4A through 4J show waveforms and signals produced at certain points in the object identification system.

FIGS. 5A through 5H are different signals produced during identification checking.

Referring now to FIG. 1, a signal repeating device is attached to each Object such as an automobile, 0r a railroad car, which is to be identified. Each signal repeating device 10 may have up to four piezoelectric elements connected in parallel with capacitor 12 to pickup antenna 13. The piezoelectric elements are selected from four different frequencies from 450 kc. through 480 kc. as indicated in the time chart shown in FIG. 4F. The specific identification device 10 shown only has two piezoelectric elements 15 and 17 connected therein.

The piezoelectric elements may be small discs of lead zirconate-titanate, or barium titanate. They may also be constructed of other materials which have a piezoelectric effect. Piezoelectric elements of lead zirconate-titanate have a resonant frequency tolerance within 0.1% from 40 C. to 85 C. The resonant frequency is estimated to change no more than i0.2% in l() years. The piezoelectric elements of lead zirconate-titanate have a minimum impedance of approximately 15 ohms at resonance. At a non-resonance frequency, their impedance is of the order of 100 ohms.

Each signal repeating device 10 therefore has a low impedance at the frequencies of the piezoelectric devices connected into the pickup antenna 13, and a high impedance at the other frequencies. Each signal repeating device 10 may be coded in binary form to represent an identif'ication number identifying the Object to which the signal repeating device 10 is connected. Each piezoelectric element has a high Q so that there is a slow decay of oscillation.

A sweep generator and voltage controlled oscillator 19 generates a signal from approximately 480 kc. to 450 kc. as shown in FIG. 4A which is applied to a multiplexer 21. The multiplexer contains four piezoelectric elements with a piezoelectric element at 450 kc., at 460 kc., at 470 kc., and at 480 kc. Thus when the multiplexer 21 is driven by the voltage controlled oscillator 19, the multiplexer produces a signal at a predetermined time in synchronism with the production of a corresponding frequency by the voltage controlled oscillator 19. The multiplexer thus produces a signal at 480 kc., 47() kc., 460 kc., and 450 kc. as shown in FIG. 4B.

The signal from the voltage controlled oscillator 19 is also applied directly to the transmitter 23 so that transmitter 23 can transmit the signal from the voltage controlled oscillator 19 when the transmitter 23 is turned on.

The signal from the multiplexer 21 as shown in FIG. 4B is applied to a sequencing circuit 25 which will turn on the transmitter for 500 microseconds as shown in FIG. 4C at the start of each of the four specific frequencies. The receiver 27 is turned off at the same time for 500 microseconds. At the end of 500 microseconds the signal from the sequencing circuit 25 will reverse as shown in FIG. 4C, turn the transmitter 23 off, and turn the receiver Z7 on. At the end of the 150 microsecond period the receiver is again turned ofi, and the transmitter 23 turned FIG. 4E shows the response at one specific frequency from a signal repeating identification device when a piezoelectric element is present at the 480 kc. and 460 kc. frequencies. Note that after the transmitter is turned off as indicated in FIG. 4D, that the response from the selected piezoelectric elements in the signal repeating identification device continues to ring. The response from the selected piezoelectric elements is an exponentially damped sinusoidal signal which decays and continues to ring after the transmitter 23 is turned ofi". The receiver 27 is turned on during this period of time as shown in FIG. 4G so that if there is a piezoelectric element at that particular frequency the receiver 27 will receive the refiected output from the particular piezoelectric element.

The output from the receiver 27 is applied through an amplifier 31 to a discriminator 32. The output from a local oscillator 19 which produces a sweep signal 15 kc. out of phase with that sweep signal supplied to transmitter 23 is also applied to the discriminator 32. The output from the discriminator 32 is applied to a digital detector 33, with the output from the digital detector applied to counter 34, and the output from counter 34 applied to the storage register 3S.

Referring now to FIG. 1 for a description of the operation of the invention, a signal repeating identification de vice 10, attached to the object to be identified, has two piezoelectric elements 15 and 17 connected in parallel with capacitor 12 to the pickup antenna 13 with piezoelectric element 1S having a frequency response of 450 kc. and piezoelectric element 17 having a frequency response of 480 kc. To identify the object according to the coded piezoelectric elements connected in the identification device, the identification device 10 is positioned over the transmitting antenna 29 and the receiving antenna 30.

Assume for the immediate purposes of this description the voltage controlled oscillator 19 and the transmitter 23 have been turned on by the sequencing circuit through the switch 36 so that the oscillator 19 applies a varying signal starting at about 487 kc. to the transmitter 23, and the transmitter transmits that signal from transmitting antenna 29 as shown in FIG. 4D.

The receiver 27 is turned off during this time by the negative signal from the sequencing circuit 25 through the switch 36.

There is a piezoelectric element at the 480 kc. frequency so that a signal is refiected from the signal repeating device at 480 kc. as shown in FIG. 4E.

The transmitter 23 is turned ofi` after 500 microseconds as shown in FIG. 4D, and the receiver 27 turned on so that the receiver 28 can receive the signal reflected from the piezoelectric element in the signal repeating device 10 at the 480 kc. frequency. The piezoelectric element at the 480 kc. signal continues to ring in an exponentially damped sinusoidal signal which decays during the microsecond period of time that the transmitter 23 is turned off, and the receiver 27 is turned on.

The 480 kc. signal received by the receiver 27 is applied through amplifier 31 to discriminator 32 where a local sweep frequency from oscillator 19, which is 15 kc. different from the transmitted sweep frequency, is also applied at the same time.

The resulting 15 kc. difference frequency is applied to a digital detector 33. This detector performs two functions. First, it gives a digital signal out which switches on and off at the frequency of the input, provided the input frequency is in the proper range of frequency. Secondly, the digital detector gives a switching output only if the input signal is a true AC signal, thereby eliminating extraneous noise signals. The input must follow a positive voltagezero volts-negative volts-zero voltspositive sequence. A proper input sequence is shown in FIG. 5A, and the corresponding output FIG. 5B. If there is noise superimposed on the signal, the input will not follow the proper voltage sequence, this is because a noise signal (FIG. 5C) and a legitimate signal (FIG. 5A) will add together to give a waveform as in FIG. 5D. This waveform does not follow the proper voltage sequence, so the output of the digital detector (5E) will not switch at the 15 kc. rate, and so will be rejected by the digital logic 34 as not being of the correct frequency.

The noise could also add an eXtra pulse on the output, if the noise burst were short. Such a noise burst is shown in FIG. F. If such a noise burst occurred, the input to the digital detector 33 would look as in 5G. The digital detector output (5I-l) will have extra pulses. This signal will also be rejected by the digital logic 34 as being too high in frequency. Noise alone (without any signal) cannot give a proper output since most electrical noise is of a random nature and is made up of many frequency components. The probability of a noise burst occurring with exactly the correct frequency and amplitude is very low. Therefore the digital detector circuit output waveform allows the digital logc 34 to reject input signals which have noise, either noise alone or superimposed with signal. By making repetitive sweeps of RF while the designator 13 is over the antennas 29, 30, and storing all legitimate signals in the storage register, eventually the entire code from the designator will be stored, even in the presence of a high electrical ambient noise.

The digital logic eliminates noise readings by measuring the half cycle times of the digital detector output and counting the number of half cycles in the 150 MS receiver on time. Since the tolerances in the piezoelectric elements cause a variation in the received frequency out of the discriminator 32 to vary over a range of from 12 to 18 kc., the number of half cycles counted will vary between crystals. Therefore the digital logic accepts a narrow range of half -cycle counts as legitimate signals. If the number of counts falls outside this range, the received signal is rejected as noise.

The legitimate signals passed by the digital logic are temporarily stored in the storage register and not read out until either (a) the designator signals cease for more than one sweep, indicating the designator is gone, or (b) after a iixed number of sweeps.

The reilected signal from the piezoelectric element at 480 kc. has then been reilected, and it has been determined that the signal is a correct signal.

Detailed description Signals from the receiver 27 are applied to terminals 41 and 43 in FIG. 2 to a primary winding 45. The signals from primary winding 45 are induced into secondary winding 47 and 49 With secondary winding 47 connected to the collector of NPN transistor 51 and the secondary winding 49 connected to the collector of NPN transistor 53.

Input terminals 55 and 57 are connected to a primary winding 59 from a chopper input which produces a signal kc. diterent than the transmitted sweep frequency signal input to primary Winding 45 and consequently the primary winding 59 induces a signal into secondary windings 61 and 63. The secondary winding 61 is connected through resistor 65 to the base of NPN transistor 51 and through the resistor 67 to the base of NPN transistor 69. Secondary winding 63 is connected through resistor 71 to the base of transistor 53 and through resistor 73 to the base of transistor 75. The collectors of transistors 69 and 75 are connected to each other at point 77 and then connected through a iilter 79 to a summing point 81 which is in turn connected to the bases of transistors 83 and 85. A poi-nt 87 connecting the secondary winding 47 and 49 is connected through a capacitor 93 to the connecting line the emitter of NPN transistor- 83 and the emitter of PNP transistor 85. A point 91 between tilter 79 and point 81 is connected through a capacitor 93 to the connecting line connecting points 87 and 89 and connected through resistor 95 to the collector of NPN transistor 97. The base of transistor 97 is connected through resistor 99 to the +12 volt bus and connected through diodes 101 and 103 to a terminal point 105 which is connected to the blanking circuit which turns this circuit off when the transmitter is turned on. A point 107 on the connecting line between points 87 and 89 is connected through resistor 109 to 6 the +12 volt common bus and through parallel resistor 111 and capacitor 113 to the zero volt common bus 115.

The collector of transistor 83 is connected to resistor 117 to the base of PNP transistor 119 and through the resistor 121 to the +12 volt common bus 123 the collector of PNP transistor is connected through resistor 125 to the base of NPN transistor 127 and then on through resistor 129 to the zero volt common bus 115. The collector of PNP transistor 85 is also connected through resistor 131 to the anode of diode 133 with the cathode of diode 133 connected to a terminal 135. Cathode of diode 133 is also connected to the cathode of diode 137 with the anode of diode 137 connected through resistor 139 to the collector of PNP transistor 119. The collector of transistor 119 is also connected through resistor 141 to the base of NPN transistor 143 and through resistor 145 to the zero volt common bus 115. The collector of transistor 127 is connected through resistor 147 and resistor 141 to the collector of transistor 119. The collector of transistor 85 is also connected through resistor 125 and resistor 151 to the collector of transistor 143. The collector of transistor 127 is connected directly to output terminal 153 and the collector of transistor 143 is connected to output terminal 155. The collector of transistor 127 is also connected through resistor 157 to the +12 volt common bus 123 and the collector of transistor 143 is also connected to the resistor 159 to the +12 volt common bus 123.

The common point between secondary windings 47 and 49 is also connected directly to an automatic gain control circuit consisting of NPN transistor 161 and PNP transistor 163. The common point 87 is connected directly to the emitter of NPN transistor 161 with the base of transistor 161 connected through a series of diodes 164 to 167 to an automatic gain control input terminal 169. The base of transistor 161. is also connected through a resistor 171 to the line from terminal 87. The collector of transistor 161 is connected to resistor 173 to the base of PNP transistor 163. The emitter of PNP transistor 163 is connected through a resistor 175 to the +12 volt common -bus 123 and back to its base through resistor 177 and connected through capaeitor 179 to the common bus 115. The collector of transistor 163 is connected through resistor 181 to the automatic gain control output terminal 183 and to capacitor 185 to the common bus 115. Terminals 153 and 155 in FIG. 2 are connected to terminals 187 and 189, respectively in FIG. 3. Terminal 135 in FIG. 2 is connected to terminal 191 in FIG. 3. Terminal 189 in FIG. 3 is connected to a single shot 193 which is in turn connected to an AND circuit 195 with the output connected to time delay 197. The output time delay 197 is connected to AND circuit 199 and to single shot 201 the input terminal 189 is also connected to an input of AND circuit 199. The input terminal 187 is connected to one terminal of AND circuit 203 and to single shot 205 the output from single shot 205 is connected to AND circuit 207 with the output from AND circuit 207 connected to time delay 209. Time delays 197 and 209 are both set time out at the end of a 15 microsecond period. The output from time delay 209 is applied to AND circuit 203 and to one single shot 201. The input from terminal 191 is connected to dip-flop 211 with the outputs from 211 connected to AND circuits 195 and 207. The output from one single shot 201 is inverted by inverting OR 213 and applied to time delay V215, the output from time delay 215 is applied to AND circuits 199 and 201. The outputs for both AND circuits 199 and 201 are applied to a counter circuit 217 with the outputs from that counter circuit 217 applied to storage register 219.

Referring now to FIG. 2 the signal from the receiver received that primary winding 45 is applied to secondary winding 47 and 49 at the same time that a local sweep frequency is applied to primary Winding l45. The frequency of the signal applied to primary winding 59 is 15 kc. different from that applied to the transmitter so that there is a difference of 15 kc. between the signal received by the receiver and applied to the receiver primary 45 and that applied to the primary winding 59 by the local oscillator. Transistors 51, 69, 71, and 75, driven by the secondary windings 61 and 63 switch the signal on and off from 47 and 49 at the local oscillator frequency, producing an output signal at terminal 77 which is a l5 kc. signal indicating the l kc. dilerence between the local oscillator input at primary winding 59 and the signal input at primary winding 45. Transistors chop the incoming signal on and off with the two top transistors 51 and 69 turning on and the bottom transistors 53 and 75 turning7 off simultaneously at the local oscillator frequency.

FIG. 4G shows the output signal from the receiver. The output is shown with two piezoelectric elements present in numbers 2 and 4. A local oscillator input is shown in FIG. 4H. With resulting 15 kc. difference out of the mixer output shown in FIG. 4I. Mixer output of course is a kc. signal when there is a legitimate signal applied to primary winding 45. Filter 79 and 93 are low frequency lters which will lter out any signal above kc.

The blanking signal applied to terminal 105 will cause the output from the mixer 46 to turn off during the transmit time. This is a second security precaution as the receiver is also turned off during transmit time. During the receiver time when the receiver is turned on, the signal applied to blanking circuit input 105 will allow the l5 kc. differences to be applied to the discriminator 48.

The input through filter 79 is applied at point S1 which applies a signal to the bases of transistors 83 and 85 is a burst of a 15 kc. pulse when there is a legitimate signal in the receiver. The positive half cycle applied to the base of transistor 83 will turn that transistor on so that the collector of transistor 83 will apply a signal to the base of transistor 119 turning that transistor on which in turn applies a signal to the base of transistor 143 to turn that transistor on thus setting the Hip-Hop consisting of transistors 127 and 143 on. Transistor 143 turned on causes a signal to be applied from terminal 155 in FIG. 2 to terminal 189 in FIG. 3. The negative half-cycle applied at point 81 is applied to the base of the transistor turning that transistor on so that a signal is applied to the base of transistor 127 resetting the flip-flop by turning transistor 127 on and causing an output signal from terminal 153 to be applied to terminal 137 in FIG. 3. The square wave output shown in FIG. 4J from terminals 153 and 155 in FIG. 2 is applied to terminals 189 and 187 respectively in FIG. 3. The output from terminal 135 is applied to terminal 191 in FIG. 3.

It should be pointed out the tolerances in the crystals in the signal repeating device shown in FIG. 1 may vary so that the signal which is a legitimate signal out of the discriminator shown in FIG. 2 may vary from a l2 kc. to an 18 kc. signal. Therefore it should be timed so that the digital defection circuit shown in FIG. 3 will determine that a valid identification signal is received if the signal varies between l2 kc. and 18 kc. The identification detection circuit shown in FIG. 3 therefore should accept as a valid signal such signals and reject signals which are longer than 18 kc. or shorter than 12 kc.

The positive half-cycle of the square wave applied to terminal 189 and applied to single shot 193 causes single shot 193 to apply a positive signal at that time to AND circuit 195. At the same time a signal is applied to flipflop 211 causing nip-flop 211 to apply signal to AND circuit 195, which applies a signal which is zero going to time delay 197. Time delay 197 times out after 18 kc. signal period which is approximately microseconds and applies a positive signal to AND circuit 199. Assuming that a valid signal has been applied to the identication circuit time delay circuit 197 times out indicating that the signal is not too high a frequency. Signal from time delay circuit 197 is also applied to one shot 201 applying the inverted signal through 213 to time delay 215 so that time delay 21S, after a short period, applies a signal to AND circuits 203 and 199. AND circuit 199, with positive signals applied to all the terminals, applies a signal to counter 217. A negative going portion of the square Wave which is applied to the terminal 187 is applied through single shot 205 and AND circuit 207 which has a signal applied there to it at this time due to the fact that ip-iop 211 is applied a signal to AND circuit 207, AND circuit 207 applies a signal to time delay 209. Time delay 209 is again as in the case of time delay 197 instructed to time out after an 18 kc. interval applying its signal to AND circuit 203 and to one shot 201. One shot 201 applies its signal to inverting OR 213 to time delay 215 which times out shortly applying its signal to AND circuit 203 so that AND circuit 203 with signals applied to all three inputs applies its signal tol counter 217.

We have therefore described how a positive going kc. and a negative going kc. signal have caused two pulses to be applied to counter 217.

This has indicated that the signal is not less than 12 kc.

The outputs from AND circuits 203 and 199 are applied to a counter circuit 217 so that it will pass one signal when it receives four or more signals from AND circuits 199 and 203 but will pass no signal when it receives one, two, three signals. Thus when four or more input pulses are applied to counter 217 it will store in storage register 219 a signal indicating that a valid identification bit has been received.

This is accomplished because the receiver is on for approximately microseconds and if the signal is 12 kc. to 18 kc. or above there will be four, five or six and if it is 12 kc. or below, there will be one to three counts received in counter 217. This is because during a 12 kc. period there is a time period of 8O microseconds. The out puts from AND circuits 199 and 203 are half-cycles so that for a 12 kc. signal there would be three half-cycles of 40 microseconds each which would equal 12() microseconds. For a lower signal rate for instance 6 kc.s there would be a count of two in the counter indicating that a signal received is below 12 kc. and it should be rejected. For a higher signal rate, time delays 197 and 209 Will not time out and no counts result.

We claim:

1. In an identification interrogation system comprising a signal repeating device associated with each object to be identified; each signal repeating device having a plurality of selected piezoelectric elements, each of a different preselected frequency response; means for interrogating said signal repeating devices with a signal sweeping a frequency range covering the frequency range of said signal repeating devices for a predetermined period of time during the preselected frequency responses of each of said plurality of selected piezoelectric elements to obtain a response of frequency-identified signals; means for generating a second sweep signal which is a predetermined frequency different from said interrogating signai; and means for receiving the response of frequency-identified signals and said second sweep signal and producing a predetermined dierence signal when there is a frequencyidentified signal reflected from said signal repeating device.

2. The invention as claimed in claim 1 wherein detection means are provided to determine if the predetermined difference frequency produced by said receiving means is within predetermined limits indicating that the difference signal is a proper frequency.

3. The invention as claimed in claim 2 wherein said receiving means for receiving the response of frequencyidentified signals is turned on for a second predetermined period of time during the preselected frequency responses of each of said plurality of selected piezoelectric elements.

4. The invention as claimed in claim 1 wherein said receiving means receives the response of frequency-identfied signals, means for discriminating between said response of frequency-identified signals and said second sweep signal to produce an AC difference frequency, and means for checking that the output difference frequency from said discriminating means is an AC frequency.

5. The invention as claimed in claim 4 wherein counting means count the half cycles output difference frequency from said discriminating means to determine that the count is within predetermined limits.

6. The invention as claimed in claim 1 wherein said receiving means receives the response of frequency-identil'led signals, means are provided for discriminating between said response of frequency-identified signals and said second sweep signal to produce an AC difference frequency, and means are provided for timing each half cycle of said AC difference frequency to determine that the AC difference frequency is within predetermined limits.

7. The invention as claimed in claim 6 wherein counting means are provided to count the half cycles of the AC difference frequency to determine that the count is within predetermined limits.

8. In an identification interrogation system comprising a signal repeating device associated with each object to be identified; each signal repeating device having a plurality of selected elements having a high Q, each of a diiferent preselected frequency response; means for interrogating said signal repeating devices with a signal sweeping a frequency range covering the frequency range of said signal repeating devices for a predetermined period of time during the preselected frequency responses of each of said plurality of selected elements having a high Q to obtain a response of frequency-identied signals; means for generating a second sweep signal which is a predetermined frequency different from said interrogating signal; and means for receiving the response of frequency-identified signals and said second sweep signal and producing a predetermined ditference signal when there is a frequencyidentied signal reflected from said signal repeating device.

References Cited UNITED STATES PATENTS 2/1965 Davis et al. 343-6.5 9/1965 Davis et al. 343--6.5 

